1. Field of the Invention
The present invention relates to an ultrasonic inspection method, i.e., one type of nondestructive inspection techniques. More particularly, it relates to the ultrasonic inspection method and equipment therefor using an array-probe ultrasonic sensor.
2. Description of the Related Art
In the ultrasonic inspection methods intended for dealing with various types of structural materials as the inspection targets, from conventionally, detecting a defect is performed as follows: Namely, an ultrasonic sensor including a single element is used for transmission and reception of ultrasonic wave. Then, an ultrasonic signal reflected by a defect or the like inside an inspection target is detected. Finally, the defect is detected based on the propagation time and position of the ultrasonic sensor.
At this time, the ultrasonic sensor is caused to displace, then determining positions at which the reflected waves from the defect can be acquired. Moreover, size of the defect is identified by an integrating operation of differences in reception times of the reflected waves from the bottom surface (i.e., boundary surface at farther distance) of the inspection target or the surface (i.e., boundary surface at nearer distance) thereof, and material ultrasonic-wave velocity (i.e., ultrasonic-wave velocity inside material of the inspection target).
This method is often used for common-purpose defect inspections, because its operation principle is easy and straightforward, and because the equipment therefor is functional enough to be comparatively simple. In this method, however, the defect must be evaluated only from the reception times of the reflected ultrasonic waves after the reflected ultrasonic waves are measured. This complexity has required a skilled and proficient inspector for implementing the high-accuracy inspection, and also has necessitated a tremendous amount of time for completing the measurement.
Meanwhile, in recent years, as known as methods such as the phased array method and synthetic aperture method, the ultrasonic inspection methods have been developed which perform the inspection by allowing the inside of an inspection target to be imaged with high accuracy (Refer to, e.g., Non-Patent Document 1).
Here, first, the phased array method is a method based on the following principle: Namely, using the so-called array-probe ultrasonic sensor where a plurality of piezoelectric vibration elements are arranged, wavefronts of ultrasonic waves transmitted from the respective piezoelectric vibration elements interfere with each other. Then, the wavefronts will propagate in such a manner that the wavefronts have formed a superimposed wavefront as a result of the interference. Accordingly, by exercising a delay control over ultrasonic-waves transmission timings of the respective piezoelectric vibration elements to shift the respective timings with each other, it becomes possible to control incident angles of the ultrasonic waves and thereby to converge the ultrasonic waves.
Also, at the reception time of the ultrasonic waves as well, reflected ultrasonic waves received at the respective piezoelectric vibration elements are added n the manner where the reflected ultrasonic waves are sifted with each other. Similarly to the transmission time, this addition makes it possible to control reception incident angles of the ultrasonic waves, and to receive the ultrasonic waves in the manner where the focusing is achieved.
Moreover, as this phased array method, there has been generally known the linear scan scheme for scanning the piezoelectric vibration elements linearly, or the sectorial scan scheme for changing the transmission and reception directions of the ultrasonic waves in a sector-shaped configuration. In any case of the above-described schemes, it is possible to scan the ultrasonic waves at high speed without displacing the ultrasonic sensor, and to arbitrarily control incident angles of the ultrasonic waves and position of the focal depth without exchanging the ultrasonic sensor. Accordingly, these schemes can be said to be technologies which allow implementation of the high-speed and high-accuracy inspection.
On the other hand, the synthetic aperture method is a method based on the following principle: Namely, when a reflected ultrasonic signal is received after the ultrasonic wave is transmitted in such a manner that the ultrasonic wave will diffuse widely inside an inspection target, position of a defect, which turns out to become sound source of the reflected ultrasonic wave received, exists on an arc whose center is position of a piezoelectric vibration element which had transmitted the ultrasonic wave and has received the reflected ultrasonic wave, and whose radius is equal to propagation distance of the reflected ultrasonic wave. Here, while sequentially changing position of the piezoelectric vibration element, transmission and reception of the ultrasonic waves are performed. Then, computational operation for reception waveforms at the respective positions is performed on an electronic computer, thereby extending the reception waveforms in an arc-like configuration. On account of the above-described principle, this operation concentrates intersection points of the arcs on the existence position of the defect which becomes the reflection source of the ultrasonic waves. As a consequence, it becomes possible to identify the position of the defect.
From an actual and practical point-of-view, the synthetic aperture method is a technology for implementing the high-resolution imaging by using position of the ultrasonic sensor and an ultrasonic waveform signal at this position, and applying the computational-operation processing thereto on an electronic computer. Concerning contents of the computational-operation processing at this time, the description has been given in Non-Patent Document 1:
[Non-Patent Document 1] coauthors: Rinsei Kondo, Yoshimasa Ohashi, and Toshiro Minomori, “Digital Signal Processing in Measurements and Sensors”, pp. 143-186, Digital Signal Processing Series Vol. 12, published on May 20, 1993 by Syoukodo Publishing Inc.